1. Field of Invention
This invention relates to high voltage direct current (HVDC) power transmission systems.
More specifically, the invention relates to a new and improved control for HVDC power converters and method of using the same for optimizing power transfer through HVDC power transmission systems from the rectifier end during both normal operating conditions and in the presence of a severe A.C. system fault.
2. Background Problem
U.S. Pat. No. 3,701,938 issued Oct. 31, 1972 for a "Minimum Valve Voltage Method and Control Apparatus For HVDC Power Converters" assigned to the General Electric Company, describes a known form of control technique and apparatus for assuring that a minimum valve voltage exists across the thyristor valve of the HVDC power converter in advance of supplying gating-on firing pulse signals to the gate drive of the thyristors. The control described in U.S. Pat. No. 3,701,938, upon a disturbance occurring on the A.C. supply system of the HVDC power transmission system, operates to immediately drive the minimum firing angle .alpha..sub.min out to some increased angle of delay that is designed to be sufficient to assure adequate firing voltage across the thyristor valves of the power converter even in the presence of A.C. system disturbances. After the passage of the A.C. system disturbance, the control allows the .alpha..sub.min limit to relax back to its original minimum limit setting after some predetermined period of time gauged to allow adequate time for the A.C. system disturbance to pass and for the system to return to normal operating conditions. This known control and technique, while adequate for certain types of A.C. system disturbances, such as the appearance of undesired harmonics on the A.C. system voltage waveform, does have certain shortcomings due to its effect on system operation as explained in the following paragraphs.
There are two basic approaches in the control of the firing of HVDC power converters, and each has its advantages and disadvantages. An ideal firing system would combine the advantages of both and hopefully eliminate the disadvantages. One of these known control techniques controls the firing of each thyristor valve individually and obtains best results where the multiphase alternating current supply system voltage waveforms are not symmetrical. However, this control technique tends to generate noncharacteristic harmonics, which accentuate filtering problems in order to avoid telephonic and radio interference. Since the multi-phase A.C. supply system employed in HVDC power transmission schemes normally provides symmetrical voltage waveforms, the other basic control technique assumes a symmetrical supply system and advances or retards all firing of the HVDC thyristor valves together so as to maintain essentially equidistant-spaced intervals between conduction for the different valves.
With the equidistant-spaced firing system, some voltage normally is maintained on each valve of the power converter whether it be a six-pulse converter, a twelve-pulse converter, etc. so that the D.C. link can continue to transmit power. It is desirable that as much power as possible be transmitted in order to maintain stability of the A.C. system. However, as shown in FIGS. 2 and 3 of the drawings, with such an arrangement, the A.C. system becomes highly unsymmetrical both in magnitude and in phase in the presence of a close-by single-phase fault at the rectifier end of the HVDC power transmission system. This dis-symmetry in the A.C. system voltage waveforms is detected by a minimum valve voltage detector such as that described in the above reference U.S. Pat. No. 3,701,938, the teachings of which are hereby incorporated in their entirety by reference, which attempts to maintain operation of the system by retarding the firing angle of all of the thyristor valves. For the best case condition, as shown in FIG. 2, the firing angle for all of the valves must be retarded by at least 30.degree., and for the worst case situation, the firing angles must be retarded by about 40.degree.. This resulting action on the part of the minimum firing voltage control circuit produces such a large drop in the HVDC link line voltage that only about 30% power can be transmitted under such conditions. Furthermore, this large drop in the rectifier voltage makes it probable that the converter current may fall to zero due to the discharge of the D.C. filter element employed in such HVDC transmission links. If this happens, it becomes necessary to close the by-pass switch and restart the HVDC transmission link thereby resulting in a period of several cycles when there is no power transmission. To overcome this problem, the present invention was devised.